Crossbows have an extensive history dating back many centuries to medieval times. Samples of crossbows and written descriptions of crossbows dating back to medieval times may be found in many museums throughout the world. The modem crossbows currently in use have evolved from the crossbows used centuries ago.
There are two well accepted methods for launching an arrow from a modern crossbow. One method employs a track type crossbow design. The other method employs a trackless design.
In the track type crossbow design, an arrow shaft rests in a track located in the stock of the crossbow in the full drawn cocked position. The arrow is launched from the crossbow by being pushed down the track with the bowstring and the arrow both maintaining intimate contact with the track until the arrow has cleared the bow. The arrows used in this type of crossbow are usually blunt at the rear end of the arrow. The bowstring that propels the arrow simply pushes against the blunt end to propel the arrow from the crossbow.
In the trackless type crossbow design, the arrow is supported on an arrow rest towards the front of the arrow shaft and the rear of the arrow is supported by being nocked to the bowstring in the same manner as is used in conventional bows.
Modern crossbows, whether of the track variety or of the trackless variety, differ from medieval crossbows in part in that they make use of twin cam leveraging units located on the tips of the bow limbs.
Twin cams have progressed from simple variable leveraging units consisting of circular shapes mounted eccentrically to more complex shapes that are intended to create more energy storage for a given power stroke. As the cam profiles have become more complex in order to store more energy, it has become more critical and more difficult to synchronize one cam to the other. It is well-documented that improper cam synchronization effects arrow flight characteristics and can result in radical deviations at the nock end of the arrow, deviations that can depart from the desired straight line impulse required for the best accuracy.
The nocking problem is similar to the well known `Archers Paradox` in which the longitudinal axis of an arrow placed into the bow for launch is not in line with the plane of travel of the bow string. Thus, the arrow does not appear to be aimed in the direction of launch. When the arrow's longitudinal axis does not lay on or very close to the plane of bowstring travel it becomes necessary to carefully match the stiffness (or spine) of the arrow to the bow and the particular set-up that is being used.
The conventional crossbows of years past required that the limbs be carefully matched for spring rate so that each limb tip pulled with equal force on its end of the string. If one limb were slightly stiffer than the other, the bowstring would be pulled slightly in the direction of the stronger limb. By having the launch string push against a blunt end of the projectile, the forces that would be imparted laterally to the rear of the projectile were minimized to the point that they would not push the rear of the projectile laterally out of the track as the projectile was launched.
With the advent of compound crossbows, the same situation exists today. The problem is not so much due to a mismatch in limb spring rate as it is due to being able to accurately synchronize the rotation of the cams at each limb tip.
Much effort has been directed towards the goal of attaining better cam synchronization and solving the nocking problem. To that end, U.S. Pat. No. 4,440,142 discloses a tunable yoke system. Other patents directed toward achieving proper cam synchronization include U.S. Pat. No. 4,372,285, U.S. Pat. No. 4,909,231, U.S. Pat. No. 5,307,787, U.S. Pat. No. 5,505,185 and U.S. Pat. No. 5,515,836. In U.S. Pat. No. 5,505,185 to Miller, for example, the simple circular idler wheel of U.S. Pat No. 5,368,006 is replaced with a multi-track element capable of taking up bowstring on one side of the element at a different rate than it was paring out bowstring on the other side. Another method of cam synchronization is the Jennings micro tune system.
Many of the modem crossbow designs have adopted the compound bow technology using radically profiled cams to achieve greater energy storage. The greater peak draw weights that are attainable using crossbows as compared to conventional bows combined with the use of increased power strokes on today's crossbows as compared to older conventional crossbows, result in problems associated with non-linear loading at the nock end of the projectile which are greater than in the past. Yet, the problems have heretofore gone unrecognized.
The use of some of the newer radically profiled cams has resulted in discrepancies in cam timing. Discrepancy in cam timing on a compound crossbow will result in the cam with the most mechanical advantage at any given time pulling the attached bowstring in the direction of the advantaged cam. The bowstring in turn, will impart a horizontal force to the end of the arrow shaft at 90.degree. to the direction of the intended arrow travel.
In the case of the track type crossbow, the nock end of the arrow rests against the bowstring with a force that is equal to the launch force being exerted on the shaft of the arrow. Therefore, the force that is exerted on the arrow due to any discrepancy in cam synchronization is equal to the propelling force multiplied by the coefficient of friction between the bowstring and the end of the nock. If the force generated in this matter is significant enough, it can cause the nock end of the arrow to be displaced as it is launched down the track.
The degree of cam non-synchronization in relation to the coefficient of friction between the nock end of the projectile and the bowstring must be significant on a track type crossbow to cause a noticeable problem in arrow flight. Normally, the side forces generated by the bowstring friction against the arrow nock are resisted by the side forces the track exerts against the arrow where it is being supported and satisfactory arrow flight can be achieved.
The trackless crossbow design is more susceptible to the effects of the cams not being properly synchronized because the arrow is only supported at its front and is intimately attached to the bowstring at the rear or nock end of the arrow. In many cases, arrows supported in this manner become free of the front support prior to the rear end of the arrow clearing the bow during launch. Unfortunately, the rear end of the arrow is free to be acted upon by the external forces exerted by the bowstring as soon as it clears the trigger assembly. As a result, any cam synchronization problem that causes the bowstring to be pulled in one direction or the other during the launch of the arrow will have a tendency to displace the nock end of the arrow horizontally in the same direction. This results a corresponding degree of erratic arrow flight.
Given the adverse effects on arrow flight that can result from a lack of synchronization between twin cams on a crossbow, it would be desirable to have a crossbow that does not require synchronization and reacts in a consistent fashion during arrow launch without imparting unwanted forces to the rear end of the arrow.
For the purpose of this disclosure, all US patents and patent applications and all other publications referenced herein are incorporated herein by reference in their entirety.